From: Leslie Viljoen <leslieviljoen@...>
Date: 2008-07-04T05:40:19+09:00
Subject: Re: How to duplicate elements in array?

On 7/3/08, Chris Chris <kylejc@gmx.net> wrote:
> Hi,
>
>  something I couldn't quite figure out (sorry for posting so soon after
>  my last question)...
>
>  a = [["ABC", "30", "1"]]
>
>  I want to duplicate/copy the element. This is what I'm trying to get:
>
>  a2 = [["ABC", "30", "1"], ["ABC", "30", "1"]]
>
>  What I did was simply
>
>  a2 = a + a
>
>  => [["ABC", "30", "1"], ["ABC", "30", "1"]]
>
>  That works.
>
>  I then tried changing a single element.
>
>  a2[0][1] = "test"
>
>  puts a2.inspect
>  => [["ABC", "test", "1"], ["ABC", "test", "1"]]
>
>  Of course, I just intended to change the first value, not both of them.
>
>
>  Can anybody explain please what I can do about this?

You need to use .dup:

>> a = ["ABC", "30", "1"]
=> ["ABC", "30", "1"]
>> b = a.dup
=> ["ABC", "30", "1"]
>> b[0] = "DEF"
=> "DEF"
>> a
=> ["ABC", "30", "1"]
>> b
=> ["DEF", "30", "1"]

If you just assign an object, you get another reference to the same
object. DUP will give you a copy, although not a deep copy. That means
that if

a = [["ABC", "30", "1"]], a.dup will not give you a copy of the
element within the array - it will give you a copy of the array with
the inner element being a reference again:
>> a = [["ABC", "30", "1"]]
=> [["ABC", "30", "1"]]
>> b = a.dup
=> [["ABC", "30", "1"]]
>> b[0][0] = "DEF"
=> "DEF"
>> a
=> [["DEF", "30", "1"]]
>> b
=> [["DEF", "30", "1"]]

The easy way to do deep copies is to use Marshal.

Les